This invention relates generally to a process for preparing electroconductive members such as imaging members and electroded donor rolls or electroded bias transfer rolls. This invention particularly concerns fabrication of such electroconductive members having a conductive layer or having an integral electrode pattern wherein the electrode pattern comprises conductive structures within the insulating polymer or ceramic layer, formed by direct irradiation of the insulating material. The present invention also relates to a process for changing the electrical properties, such as conductivity, of an insulating material layer of an electroconductive member such as an imaging member, a donor roll or a bias transfer roll.
Generally, the processes of electrostatographic imaging and electrophotographic printing include the steps of charging a photoconductive imaging member to a substantially uniform potential so as to sensitize the photoconductive surface thereof. The charged portion of the photoconductive imaging member is exposed to an image of an original document being reproduced, such as a visible image of an original document being reproduced or a computer-generated image written by, for example, a raster output scanner. This records an electrostatic latent image on the photoconductive imaging member corresponding to the original document or computer-generated image. The recorded latent image is then developed by bringing oppositely charged toner particles into contact with it. This forms a toner powder image on the imaging member that is subsequently transferred to a substrate, such as paper. Finally, the toner powder image is permanently affixed to the substrate in image configuration, for example by heating and/or pressing the toner powder image.
Transfer of the latent image from the imaging member to the recording substrate is most commonly achieved by applying electrostatic force fields in a transfer nip sufficient to overcome the forces holding the toner to the imaging member and to attract most of the toner to transfer it onto the recording substrate. These transfer fields are generally provided in one of two ways, by ion emission from a transfer corona generator onto the back of the copy sheet, as described in U.S. Pat. No. 2,807,233, or by a DC charged biased transfer roller or belt rolling along the back of the copy sheet. Examples of bias roller transfer systems are described in U.S. Pat. Nos. 3,781,105, 2,807,233, 3,043,684, 3,267,840, 3,328,193, 3,598,580, 3,625,146, 3,630,591, 3,684,364, 3,691,993, and 3,702,482. Further examples of a biased transfer roller are described in U.S. Pat. Nos. 3,924,943 and 5,337,127.
A suitable developer material may be a two-component mixture of carrier particles having toner particles triboelectrically adhered thereto. The toner particles are attracted to and adhere to the electrostatic latent image to form a toner powder image on the imaging member surface. Suitable single component developers are also known. Single component developers comprise only toner particles; the particles have an electrostatic charge (for example, a triboelectric charge) so that they will be attracted to, and adhere to, the latent image on the imaging member surface.
There are various known forms of development systems for bringing toner particles to a latent image on an imaging member surface. One form includes a magnetic brush that picks up developer from a reservoir through magnetic attraction and carries the developer into proximity with the latent image. In a modification of the magnetic brush apparatus, known as hybrid development, the magnetic brush does not bring toner directly to the imaging member surface, but transfers toner to a donor roll that then carries the toner into proximity with the latent image. In single component scavengeless development, a donor roll is used with a plurality of electrode wires closely spaced from the donor roll in the development zone. An AC voltage is applied to the wires to form a toner cloud in the development zone and the electrostatic fields generated by the latent image attract toner from the cloud to develop the latent image. In a hybrid scavengeless development system, the method of development with a donor roll is the same as in single component scavengeless development, except that a magnetic brush is first used to load the donor roll with toner particles. In this system, the donor roll and magnetic brush are electrically biased relative to one another; thus toner is attracted to the donor roll from the magnetic brush. The electrically biased electrode wires then detach toner from the donor roll, forming a toner cloud in the development zone and thereby developing the latent image.
In single component jumping development, an AC voltage is applied to the donor roll, causing toner to be detached from the roll and projected towards the imaging member surface. The toner is attracted by the electrostatic fields generated by the latent image and the latent image is developed. Variants of these development systems may be used with single component or two-component developers.
An electrophotographic imaging member for use in these processes may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium, or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements on photoreceptors, including narrow operating limits. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating, an adhesive layer and an optional overcoating layer.
Several of these types of photoreceptors are disclosed in, for example, U.S. Pat. Nos. 5,021,309, 5,200,286 and 5,372,904.
In multi-color electrostatographic printing, a photoconductive intermediate transfer belt is used. Rather than forming a single latent image on a photoconductive surface, successive latent images corresponding to different colors must be created. Each single color latent electrostatic image is developed with a corresponding different colored toner, and thus the process is repeated for a plurality of cycles. Each single-color toner image is then superimposed over the previously transferred single-color toner image when it is transferred to the recording substrate such as a copy sheet. This creates a multilayered toner image on the copy sheet. One way to transfer each of the several latent images is to develop one or more of the latent images on a single intermediate transfer belt, rather than on separate photoreceptors, and then transfer the latent images from the intermediate belt to the recording substrate. Such a photoconductive intermediate transfer belt is disclosed in, for example, U.S. Pat. No. 5,347,353.
Generally, as described above, a donor roll is used in many electrostatographic development systems. The donor roll is used to transport toner particles, for example, from a magnetic brush to the development zone to be applied to the surface of a photoreceptor. A purpose of the donor roll is to transfer the toner to the photoreceptor without significantly disturbing or removing toner particles already on the surface of the photoreceptor. Thus, for example, a donor roll is preferred in color imaging processes where all of the toner particles are not applied to the photoreceptor at the same time.
In order to function properly as an electroconductive member, it is necessary that the electroconductive layer or layers of the member have a specific RC time constant. The RC time constant is the product of the resistance and capacitance of the roll, and indicates the time required for charging and discharging the roll. That is, the member must be capable of dissipating an applied charge, such as an electrostatic charge, within a specified time range. Generally, the time range varies from about 0.06 to about 1.5 msec, with the time constant being defined by the resistance and capacitance of the imaging member coating or by the resistivity and dielectric constant of the coating. The electroconductive member materials, in addition to retaining or dissipating an applied charge, must also have good wear resistance, must be compatible with the toner to be used in the development system, and must have good bond strength with any substrate or other layers used in the imaging member.
The RC time constant is also important in the context of imaging members because, for example, the electrical characteristics of a layer of the imaging member must be properly adjusted to ensure imagewise transfer and development of a latent image. For example, whereas it may be necessary for one layer of the imaging member to retain a charge for a longer period of time, other layers may require faster discharge and transport of charges.
Bias transfer rolls must similarly have strictly determined electrical properties in order to properly transfer an image to the recording substrate. For example, the difficulties of successful image transfer are well known. In the pre-transfer (pre-nip) region, before the recording substrate (copy paper) contacts the latent image, if the transfer fields are too high the image is susceptible to premature transfer across the air gap, leading to decreased resolution or fuzzy images. Further, if ionization is present in the pre-nip air gap due to high fields, it may lead to strobing or other image defects, loss of transfer efficiency, and a lower latitude of system operating parameters. However, in the directly adjacent nip region itself, the transfer field should be as large as possible (greater than approximately 20 volts per micron) to achieve high transfer efficiency and stable image transfer. In the next adjacent post-nip region, at the photoconductor/copy sheet separation (stripping) area, if the transfer fields are too low hollow characters may be generated. On the other hand, improper ionization in the post-nip region may cause image instability or copy sheet detacking problems. Variations in ambient conditions, copy paper, contaminants, etc., can all affect the necessary transfer parameters. The bias roll material resistivity and paper resistivity can change greatly with humidity, etc. Further, conduction of the bias charge from the bias transfer roller is also greatly affected by the presence or absence of the copy paper between it and the imaging surface. To achieve these different transfer field parameters consistently, and with appropriate transitions, is difficult.
In the past, the properties of the electroconductive roll coatings have been achieved by using a coating material, generally a semiconducting material or a mixture of a conductive material dispersed in a binder resin, having specific electrical properties such as resistivity and dielectric constant. As necessary, conductive wiring was then applied to the roll as a separate step or steps. Thus the semiconducting material was selected to have a predetermined RC time constant. A problem with this method, however, is that small variations in material formulation and/or the coating process can result in large variations in the resultant RC time constant of the roll. As a result, reproducibility of properties may be difficult to achieve and the development process and resultant image may be adversely affected.
Furthermore, several problems exist with conventional donor rolls in many of these development systems when some toner materials are used. It has been found that for some toner materials, the tensioned electrically biased wires in self-spaced contact with the donor roll tend to vibrate. This vibration may cause non-uniform solid area development of the resultant developed image. Furthermore, there is a possibility that debris within the development system can momentarily lodge on the wires. Such debris can cause streaking of the resultant print image. Thus, it would appear to be advantageous to replace the externally located electrode wires with electrodes integral to the donor roll. In addition, the removal of electrode wires from the development zone would obviate the need for a structure to maintain tension in the wires and to position the wires within the development zone.
One such method of forming integral electrodes in a donor roll, thereby forming an electroded roll, is disclosed, for example, in U.S. Pat. No. 5,268,259 to Sypula. In Sypula, the electrodes are formed in the donor roll by a process comprising: (a) providing a cylindrically shaped insulating member; (b) coating the insulating member with a light sensitive photoresist; (c) patterning the photoresist by exposure to light, resulting in a first photoresist portion corresponding to the electrode pattern and a second photoresist portion; (d) removing the first photoresist portion, thereby exposing a portion of the insulating member; and (el depositing conductive metal on the portion of the insulating member where the first photoresist portion has been removed, resulting in an electrode pattern that is capable of being electrically biased to detach toner particles from the donor roll.
Another method for forming electrode patterns on a substrate, using an electroless process, is disclosed in U.S. Pat. No. 5,153,023 to Orlowski et al. The process allows for the formation of at least one electrically conductive path in a plastic substrate. The process comprises: (a) providing a thermoplastic substrate having a melting point below 325.degree. C.; (b) coating the substrate with a precursor of a catalyst for the electroless deposition of conductive metals, the catalyst precursor having a decomposition temperature below the melting point of the thermoplastic substrate and within the temperature range where the thermoplastic substrate softens; (c) heating the portion of the coated thermoplastic substrate corresponding to the desired conductive path to a temperature sufficient to decompose the catalyst precursor to a catalyst and soften the thermoplastic substrate; and (d) depositing conductive metal by electroless deposition on the heated portion of the thermoplastic substrate to form a conductive path. In the process, the substrate, catalyst precursor and temperature are selected such that, upon heating, the precursor decomposes to a catalyst and the thermoplastic substrate softens and at least partially melts without substantial decomposition. This softening enables the catalyst to penetrate the surface of the thermoplastic substrate and become anchored therein. The catalyst then provides nucleation sites for the subsequent electroless deposition of conductive metal. The substrate containing the electrically conductive path may be a planar member, a two-sided circuit board, or a frame or structural member in a machine such as an automatic reprographic machine, which includes office copiers, duplicators and printers.
An electrode pattern may also be formed by evaporation, sputtering, spraying conductive materials through a mask, or by electrodepositing through a previously patterned conductive surface. These and other methods are known in the art.
A process of irradiating a polymer to form patterns of permanently increased electrical conductivity is described in Schumann et al., "Permanent Increase of the Electrical Conductivity of Polymers Induced by Ultraviolet Laser Radiation," Appl. Phys. Lett., Vol. 58(5), 428-30 (4 February 1991); Phillips et al., "Sub-100 nm Lines Produced Ablation in Polyimide," Appl. Phys. Lett., Vol. 58(24), 2761-63 (17 June 1991); Phillips et al., "Submicron Electrically Conducting Wires Produced in Polyimide by Ultraviolet Laser Irradiation," Appl. Phys. Lett., Vol. 62(20), 2572-74 (17 May 1993); Srinivasan et al., "Generation of Electrically Conducting Features in Polyimide (Kapton.TM.) Films With Continuous Wave, Ultraviolet Laser Radiation," Appl. Phys. Lett., Vol. 63(24), 3382-83 (13 December 1993); Phillips et al., "Excimer-Laser-Induced Electric Conductivity in Thin-Film C.sub.60," Appl. Phys. A, Vol. 57, 105-07 (1993); and Feurer et al., "Ultraviolet Laser-induced Permanent Electrical Conductivity in Polyimide," Appl. Phys. A, Vol. 56, 275-81 (1993). The references generally discuss the formation of conducting lines (wires) in a polyimide material using cw (argon), excimer, and UV laser irradiation. The references disclose that such processes may be useful in semiconductor and integrated circuit processing applications as a means to replace the wet resist production processes. The references do not disclose application of the process to the production of electroconductive members for use in electrostatographic imaging processes.
A similar process is disclosed in N. R. Quick, "Direct Conversion of Conductors in Ceramic Substrates," ISHM Proceedings (1990). The disclosed process uses a Nd:YAG laser system with an emission wavelength of 1064 nm to generate nonmetallic electrode lines in alpha-silicon carbide and aluminum nitride substrates.
A problem with the methods currently used to form electrode patterns in imaging members such as electroded donor rolls and bias transfer rolls is the difficulty in implementing those processes on a commercial scale. For example, the multi-step nature of the processes, combined with the exacting product specifications and process control required, make the processes costly and difficult to implement. The processes also raise the problem of defects and contamination due to the numerous contacting steps necessary in the processes. A need therefore continues to exist in the field for improved processes for forming electroconductive members in general, and particularly for forming electrode patterns on members such as donor and bias transfer rolls.
There also continues to be a need in the art for a means to alter the electrical properties such as conductivity of an insulating polymer or ceramic layer of an electroconductive member so as to set and tune the RC time constant of the member. This is necessary to provide desired charge dissipation and other properties of the electroconductive member. In addition to being able to establish set properties in the electroconductive member layer, there is a need for a process that can provide more reproducible and constant results from one member to the next in the production process.